During a procedure of approach to a landing runway (also called “runway”) of an airport, an aircraft can be guided for its landing on the runway by a radio navigation system. This radio navigation system can correspond to an instrument landing system (ILS, standing for “Instrument Landing System” in English). The ILS system comprises at least a transmitting station capable of transmitting guidance signals allowing the aircraft to come closer to a reference approach trajectory. Among these signals, there is notably a vertical guidance signal, otherwise called a descent alignment signal, making it possible for the aircraft to know the vertical deviation it has with respect to the reference approach trajectory. This vertical guidance signal is generally called the “Glide signal” or the “Glide slope signal”. There is also a lateral guidance signal, otherwise called a runway alignment signal, making it possible for the aircraft to know then lateral deviation that it has with respect to the reference approach trajectory. This lateral guidance signal is generally called the “Localizer signal”.
There are three categories of ILS systems. One of said categories, notably category I (CAT I), corresponds to a procedure making it possible for the aircraft to descent automatically down to a decision altitude of 200 feet (61 m) on condition that the runway visual range (RVR, standing for “Runway Visual Range” in English) is sufficient. If the pilot can see the approach lights of the runway, the aircraft can land under manual control. If not, a go-around must be initiated.
There is also an enhanced vision system (EVS, standing for “Enhanced Vision System” in English) which makes it possible to reduce the decision altitude from 200 feet (61 m) to 100 feet (30.5 m) with the help of infrared sensors. The pilot can then control the aircraft manually starting from 200 feet. If the infrared image shows the approach lights of the runway, the pilot can steer the aircraft based on the information provided by the infrared sensors. Below 100 feet, the sensors are deactivated by the pilot who must then check if the runway visual range is sufficient. If not, a go-around must be initiated.
However, this enhanced vision system necessitates the use of a head-up display (“head-up display” being the English term) upon which the pilot has to interpret the images coming from the infrared sensors.